Target tracking is the process of calculating the path or track of a moving object by monitoring its current position and using the data obtained from this monitoring to project the targets future position. Monitoring the position of an object may be performed either actively or passively. Active monitoring requires sending out a signal from a station and receiving reflections (or hits) of those signals from an on-coming target. Passive monitoring requires that a station receive a signal (return) sent out by the target. Either method can make use of multiple stations in order that the position be more accurately determined by triangulation methods. A common example of target tracking is the process of air traffic control: radar stations send out signals and receive reflections from on-coming planes (targets), thereby determining the flight path (track) of the plane.
In many applications, multiple targets must be tracked. The more common situation is that multiple targets are moving in relatively close proximity to the other and the path of each must be distinguished and determined. Where multiple observations of multiple targets are being received from multiple stations, correctly calculating the track of each target requires that each observation be correctly associated with a specific target. Where the target paths are closely spaced, matching observations to targets presents a significant problem. This is clearly illustrated in FIG. 1A. FIG. 1A, a projection on a plane extending roughly perpendicular to the surface of the earth, shows three previously calculated tracks, B.sub.1, B.sub.2, and B.sub.3, for targets T.sub.1, T.sub.2, and T.sub.3 (not shown), respectively. New returns from the observation stations, satellites S1 and S2 are indicated by points O.sub.i. Six partial tracks calculated from these returns are shown as R.sub.1 through R.sub.6. As can be seen, the partial track R.sub.2 could be associated with either B.sub.1 or B.sub.2. If the partial track was incorrectly determined to correlate (i.e. was incorrectly associated) with B.sub.2, then the track that is projected from the collection of observation points in B.sub.2 and R.sub.2 could diverge from the actual path of the target. In the worst case, the calculated track could terminate.
FIG. 1A also illustrates another problem. When returns are received from multiple stations (or multiple pairs of stations), it is common in the art to take all combinations of returns and attempt to create small track pieces such as R.sub.5 and R.sub.6. As can be seen in the figure, return O.sub.56 was used to calculate both R.sub.5 and R.sub.6. In reality, of course, O.sub.56 could only have come from one target and should only be used once in combination with one of the three previously calculated tracks. Accordingly, if R.sub.5 is correlated with B.sub.2, the conclusion is that O.sub.56 came from T.sub.2 and should not be used in any calculations that involve projected tracks for either of the other targets. Further, in the case illustrated, R.sub.6 should not be correlated with any of the tracks, since one of its observations was already used in the correlation of R.sub.5 and B.sub.2. This type of problem is generally referred to as "duplicate returns."
It should be noted that FIG. 1A, and in fact, none of the figures, are drawn to scale. In reality, the area scanned by the satellites is orders of magnitude greater than the length of the partial tracks.